Steel sheet for hot stamping and method for producing steel sheet for hot stamping, and hot stamp formed body

ABSTRACT

A steel sheet for hot stamping includes a composition including at least, in mass %, C: 0.100% to 0.600%, Si: 0.50% to 3.00%, Mn: 1.20% to 4.00%, Ti: 0.005% to 0.100%, B: 0.0005% to 0.0100%, P: 0.100% or less, S: 0.0001% to 0.0100%, Al: 0.005% to 1.000%, and N: 0.0100% or less, with a balance of Fe and impurities, surface roughness of the steel sheet satisfies Rz&gt;2.5 μm, and 50 mg/m 2  to 1500 mg/m 2  of coating oil is applied to a surface.

TECHNICAL FIELD

The present invention relates to a steel sheet for hot stampingexcellent in scale adhesion at the time of hot stamping and a method forproducing the steel sheet for hot stamping, and a hot stamp formed bodythat is a formed body of the steel sheet for hot stamping.

BACKGROUND ART

Weight reduction of the members such as door guard bars and side membersof automobiles are being studied to cope with the recent trend ofimprovement in fuel efficiency, and in terms of a material, increase instrength of a steel sheet is promoted from the viewpoint of strength andcrash safety that should be ensured even when the thickness is reduced.Hereinafter, strength means both tensile strength and yield strength.However, formability of a material deteriorates as the strengthincreases, and therefore in order to realize reduction in weight of theabove described members, it is necessary to produce a steel sheet thatsatisfies both formability and high strength. As a method for obtaininghigh formability simultaneously with high strength, there are TRIP(TRansformation Induced Plasticity) steels taking advantage ofmartensitic transformation of retained austenite that are described inPatent Literature 1 and Patent Literature 2, and application of TRIPsteels has been expanding in recent years. In the steel, however,although deep drawability and elongation are improved at the time offorming, due to a high steel sheet strength, it has a problem of lowshape fixability of a member after press forming.

In order to form a high strength steel sheet, which is inferior informability, with good shape fixability, there is a method called hotpress that is described in Patent Literature 3 and Patent Literature 4.The method performs forming at a temperature of 200° C. to about 500° C.at which the steel sheet strength reduces. However, when forming of thehigh strength steel sheet of 780 MPa or more is considered, the methodhas problems in that even when the forming temperature is increased, thesteel sheet strength may still be high in some cases and thus forming isdifficult, and in that the steel sheet strength after forming is reducedby heating, and thus predetermined strength cannot be obtained in somecases.

As a method for solving the problems, there exists a method called hotstamping that cuts a soft steel sheet in a predetermined size,thereafter, heats the steel sheet to an austenite single phase region at800° C. or higher, thereafter performs press forming in the austenitesingle phase region as disclosed in Patent Literature 5, and thereafterperforms hardening. As a result, it is possible to manufacture a memberthat has high strength of 980 MPa or more and is excellent in shapefixability.

However, in hot stamping, a steel sheet is inserted into a heatingfurnace, or is heated to a high temperature exceeding 800° C. byelectrical heating or far-infrared heating in the atmosphere, and thushot stamping has a problem of scale generated on a steel sheet surface.A die may be worn out due to the generated scale released at the time ofhot stamping, and therefore it is required that scale adhesion should beexcellent at the time of hot stamping. As a technique that solves theseproblems, there is known a technique of restraining generation of scaleby making an atmosphere in the heating furnace a non-oxidationatmosphere in Patent Literature 6, for example. However, it is necessaryto implement atmosphere control in the heating furnace strictly, andthus facility cost increases, and productivity is reduced. Further, thesteel sheet which is taken out is exposed to the atmosphere, and thusthe technique has a problem of unavoidable formation of scale. Inaddition, in recent years, for the purpose of enhancing productivity ofhot stamping, the method for electrically heating a steel sheet in theatmosphere has been developed. At the time of heating in the atmosphere,avoidance of oxidation of the steel sheet is difficult, and thus aproblem of die wear due to loose scale at the time of hot stampingeasily becomes evident. As a result, regular repair of the die isessential.

There is known a technique of restraining wear of a die caused by loosescale by using, in hot stamping, a steel sheet with zinc plating or Alplating applied to a steel sheet surface as the steel sheet that solvesthese problems. However, since zinc plating or Al plating are meltedinto a liquid phase at the time of heating, the technique has a problemof zinc or Al adhering to the inside of the heating furnace and the dieat the time of conveyance of the steel sheet or the time of pressing. Adeposit of adhering zinc or Al has a problem of causing indentationflaws of a hot stamp formed body, and adhering to the formed body toworsen the outer appearance. Consequently, it is necessary to repair thedie regularly.

Consequently, it is required to develop a steel sheet for hot stampingin which scale does not detach at the time of hot stamping, and adhesionof a molten metal to a die does not occur.

CITATION LIST Patent Literatures

-   Patent Literature 1: Japanese Laid-open Patent Publication No.    01-230715-   Patent Literature 2: Japanese Laid-open Patent Publication No.    02-217425-   Patent Literature 3: Japanese Laid-open Patent Publication No.    2002-143935-   Patent Literature 4: Japanese Laid-open Patent Publication No.    2003-154413-   Patent Literature 5: Japanese Laid-open Patent Publication No.    2002-18531-   Patent Literature 6: Japanese Laid-open Patent Publication No.    2004-106034-   Patent Literature 7: Japanese Laid-open Patent Publication No.    2002-18531-   Patent Literature 8: Japanese Laid-open Patent Publication No.    2008-240046-   Patent Literature 9: Japanese Laid-open Patent Publication No.    2010-174302-   Patent literature 10: Japanese Laid-open Patent Publication No.    2008-214650

SUMMARY OF THE INVENTION Technical Problem

In the light of the aforementioned problems, the present invention hasan object to provide a steel sheet for hot stamping that is excellent inscale adhesion at the time of hot stamping, without an occurrence ofadhesion of a molten metal to a die, a method for manufacturing thesteel sheet for hot stamping, and a hot stamp formed body.

Solution to Problem

The present inventors have studied earnestly on methods to solve theabove described problems. As a result, with the intention to improvescale adhesion of a steel sheet, 0.50 mass % to 3.00 mass % of Si iscontained in the steel sheet, the amount of rust inhibiting oil that isapplied to the steel sheet is set to be within a range of 50 mg/m² to1500 mg/m², and surface roughness of the steel sheet is set as Rz>2.5μm. Further, an S content included in the rust inhibiting oil ispreferably set at 5 mass % or less. Thereby, it has been found thatscale adhesion at the time of heating and at the time of hot stamping isimproved. In general, enclosures in the coating oil concentrate into aninterface between a base iron and scale, and thereby deteriorate scaleadhesion. However, it has been found out that it is possible to ensurescale adhesion by using restriction on an enclosure amount, and ananchor effect using irregularities on the steel sheet surface incombination.

The present invention is made based on the above described knowledge,and the gist of the present invention is as follows.

-   (1) A steel sheet for hot stamping including a composition    containing,

in mass %,

C: 0.100% to 0.600%;

Si: 0.50% to 3.00%;

Mn: 1.20% to 4.00%;

Ti: 0.005% to 0.100%;

B: 0.0005% to 0.0100%;

P: 0.100% or less;

S: 0.0001% to 0.0100%;

Al: 0.005% to 1.000%;

N: 0.0100% or less;

Ni: 0% to 2.00%;

Cu: 0% to 2.00%;

Cr: 0% to 2.00%;

Mo: 0% to 2.00%;

Nb: 0% to 0.100%;

V: 0% to 0.100%;

W: 0% to 0.100%, and

a total of one kind or two or more kinds selected from a groupconsisting of REM, Ca, Ce and Mg: 0% to 0.0300%,

with a balance being Fe and impurities,

wherein surface roughness of the steel sheet satisfies Rz>2.5 μm, andcoating oil in an amount of 50 mg/m² to 1500 mg/m² is applied onto asurface.

-   (2) The steel sheet for hot stamping according to (1) described    above,

wherein an amount of S contained in the coating oil which is appliedonto the steel sheet is 5% or less in mass %.

-   (3) The steel sheet for hot stamping according to (1) or (2)    described above,

wherein the composition of the steel sheet contains, in mass %,

one kind or two or more kinds selected from a group consisting of

-   Ni: 0.01% to 2.00%,

Cu: 0.01% to 2.00%,

Cr: 0.01% to 2.00%,

Mo: 0.01% to 2.00%,

Nb: 0.005% to 0.100%,

V: 0.005% to 0.100%, and

W: 0.005% to 0.100%.

-   (4) The steel sheet for hot stamping according to any one of (1)    to (3) described above,

wherein the composition of the steel sheet contains, in mass %,

a total of 0.0003% to 0.0300% of one kind or two or more kinds selectedfrom the group consisting of REM, Ca, Ce and Mg.

-   (5) A method for producing a steel sheet for hot stamping, including

a step of casting a slab containing,

in mass %,

C: 0.100% to 0.600%,

Si: 0.50% to 3.00%,

Mn: 1.20% to 4.00%,

Ti: 0.005% to 0.100%,

B: 0.0005% to 0.0100%,

P: 0.100% or less,

S: 0.0001% to 0.0100%,

Al: 0.005% to 1.000%,

N: 0.0100% or less,

Ni: 0% to 2.00%,

Cu: 0% to 2.00%,

Cr: 0% to 2.00%,

Mo: 0% to 2.00%,

Nb: 0% to 0.100%,

V: 0% to 0.100%,

W: 0% to 0.100%, and

a total of one kind or two or more kinds selected from a groupconsisting of REM, Ca, Ce and Mg: 0% to 0.0300%,

with a balance being Fe and impurities, and hot rolling the slabdirectly or by allowing the slab to cool and heating the slab to obtaina hot-rolled steel sheet,

a step of pickling the hot-rolled steel sheet for seconds or more in anaqueous solution having a temperature of 80° C. to lower than 100° C.and including an inhibitor with a concentration of an acid being 3 mass% to 20 mass %, and

a step of applying a rust inhibiting oil to the steel sheet aftercarrying out the pickling,

wherein a rust inhibiting oil remaining amount on a steel sheet surfaceis limited to 50 mg/m² to 1500 mg/m².

-   (6) The method for producing a steel sheet for hot stamping    according to (5) described above, wherein the rust inhibiting oil is    applied to the hot-rolled steel sheet which has been pickled.-   (7) The method for producing a steel sheet for hot stamping    according to (5) described above, further including a step of cold    rolling the hot-rolled steel sheet which has been pickled to obtain    a cold-rolled steel sheet,

wherein the rust inhibiting oil is applied to the cold-rolled steelsheet.

-   (8) The method for producing a steel sheet for hot stamping    according to (5) described above, further including a step of cold    rolling the hot-rolled steel sheet which has been pickled, and    further performing thermal treatment in a continuous annealing    facility or a box type annealing furnace to obtain a cold-rolled    steel sheet,

wherein the rust inhibiting oil is applied to the cold-rolled steelsheet.

-   (9) The method for producing a steel sheet for hot stamping    according to any one of (5) to (8) described above,

wherein an amount of S in the rust inhibiting oil that is applied to thesteel sheet is 5% or less in mass %.

-   (10) The method for producing a steel sheet for hot stamping    according to any one of (5) to (9),

wherein a composition of the slab contains, in mass %,

one kind or two or more kinds selected from a group consisting of

Ni: 0.01% to 2.00%,

Cu: 0.01% to 2.00%,

Cr: 0.01% to 2.00%,

Mo: 0.01% to 2.00%,

Nb: 0.005% to 0.100%,

V: 0.005% to 0.100%, and

W: 0.005% to 0.100%.

-   (11) The method for producing a steel sheet for hot stamping    according to any one of (5) to (10) described above,

wherein a composition of the slab contains, in mass %,

a total of 0.0003% to 0.0300% of one kind or two or more kinds selectedfrom the group consisting of REM, Ca, Ce and Mg.

-   (12) A hot stamp formed body, including a composition containing,

in mass %,

C: 0.100% to 0.600%,

Si: 0.50% to 3.00%,

Mn: 1.20% to 4.00%,

Ti: 0.005% to 0.100%,

B: 0.0005% to 0.0100%,

P: 0.100% or less,

S: 0.0001% to 0.0100%,

Al: 0.005% to 1.000%,

N: 0.0100% or less,

Ni: 0% to 2.00%,

Cu: 0% to 2.00%,

Cr: 0% to 2.00%,

Mo: 0% to 2.00%,

Nb: 0% to 0.100%,

V: 0% to 0.100%,

W: 0% to 0.100%, and

a total of one kind or two or more kinds selected from a groupconsisting of REM, Ca, Ce and Mg: 0% to 0.0300%,

with a balance being Fe and impurities,

wherein three or more irregularities in a range of 0.2 μm to 8.0 μm indepth are present per 100 μm in an interface between scale and a baseiron, and tensile strength is 1180 MPa or more.

-   (13) The hot stamp formed body according to (12) described above,

wherein an Si oxide, FeO, Fe₃O₄ and Fe₂O₃ are included in a surface ofthe hot stamp formed body,

and a thickness of the scale is 10 μm or less.

-   (14) The hot stamp formed body according to (12) or (13) described    above,

wherein the composition of the hot stamp formed body contains, in mass%,

one kind or two or more kinds selected from a group consisting of

Ni: 0.01% to 2.00%,

Cu: 0.01% to 2.00%,

Cr: 0.01% to 2.00%,

Mo: 0.01% to 2.00%,

Nb: 0.005% to 0.100%,

V: 0.005% to 0.100%, and

W: 0.005% to 0.100%.

-   (15) The hot stamp formed body according to any one of (12) to (14)    described above,

wherein the composition of the hot stamp formed body contains, in mass%,

a total of 0.0003% to 0.0300% of one kind or two or more kinds selectedfrom the group consisting of REM, Ca, Ce and Mg.

Advantageous Effects of Invention

According to the present invention, the steel sheet for hot stampingexcellent in scale adhesion at the time of hot stamping, in whichadhesion of a molten metal to the die does not occur, the method forproducing the steel sheet for hot stamping and the hot stamp formed bodycan be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a relationship between a coating oilamount on a steel sheet and surface roughness Rz of the steel sheet.

FIG. 2 is a diagram for explaining that when an S concentration incoating oil increases, scale easily detaches.

FIG. 3 is a diagram illustrating a relationship between a pickling timeperiod and the surface roughness Rz of the steel sheet.

FIG. 4A is a photograph showing a microstructure of a surface layer of ahot-rolled steel sheet before pickling.

FIG. 4B is a photograph showing the surface layer microstructure afterpickling.

FIG. 5 is a diagram illustrating a relationship between an coating oilamount and a thickness of scale.

FIG. 6A is a photograph showing a section of a hot stamp formed bodysurface of an example of the present invention.

FIG. 6B is a photograph showing a section of a hot stamp formed bodysurface of a comparative example.

FIG. 7 is a diagram for explaining that when the surface roughness Rzbefore hot stamp thermal treatment is less than 2.5, a number density ofirregularities after hot stamp thermal treatment is less than 3.

DESCRIPTION OF EMBODIMENTS

A steel sheet for hot stamping of the present invention contains from0.5 mass % to 3.0 mass % of Si in the steel sheet, an amount of rustinhibiting oil applied to the steel sheet is in a range of 50 mg/m² to1500 mg/m², and surface roughness of the steel sheet is Rz>2.5 μm. It ispreferable that an S content contained in the rust inhibiting oil be 5mass % or less.

First of all, the reason why the present inventors paid attention to thecoating oil will be described.

With an objective of improving scale adhesion of the steel sheets towhich no plating is applied (cold-rolled steel sheets or hot-rolledsteel sheets), the present inventors have investigated the surfaceproperties of the steel sheets, and influences of various kinds oftreatment. As a result, the present inventors have found that althoughthe steel sheets after degreasing show excellent scale adhesion, scaleadhesion significantly deteriorates after rust inhibiting oil isapplied. When the present inventors investigated the relationshipbetween scale adhesion and rust inhibiting oil in more detail, it hasbeen found that when an amount of S contained as impurities in the rustinhibiting oil increases, scale tends to detach easily. It isconceivable that S in the rust inhibiting oil has an influence on scaleadhesion, although the detailed reason is unclear.

On the other hand, it is necessary to apply rust inhibiting oil such asmineral oil to a pickled hot-rolled steel sheet for hot stamping, and acold-rolled steel sheet for hot stamping after cold rolling or annealingin order to restrain rust from occurring in the period from productionto use. In particular, a steel sheet after pickling has been generallycoated with oil of more than 1500 mg/m², assuming that the period fromdelivery to a customer to use is long. When the present inventorsinvestigated the influence of the coating oil amount for the purpose ofmaking scale adhesion and rust inhibition properties compatible, thepresent inventors have found that as illustrated in FIG. 1, scaleadhesion is enhanced by strictly controlling the range of the coatingoil amount and the surface roughness of a steel sheet. The effect isexhibited by setting the coating oil amount at 50 mg/m² to 1500 mg/m². Alower limit of the coating oil amount is set at 50 mg/m², because it isdifficult to ensure excellent rust inhibition properties with thecoating oil amount less than the coating oil amount of 50 mg/m². Thelower limit of the coating oil amount is preferably 100 mg/m² or more,and more preferably 200 mg/m² or more. An upper limit is set at 1500mg/m² to obtain an effect of excellent scale adhesion. The upper limitof the coating oil amount is set at 1500 mg/m² because when the coatingoil amount exceeds 1500 mg/m², scale adhesion deteriorates. The upperlimit is preferably 1000 mg/m², is more preferably 900 mg/m², and farmore preferably is 800 mg/m². Further, coated oil on the steel sheetsurface burns at the time of heating, and therefore becomes the cause ofgenerating soot. From this, a smaller coating oil amount is morepreferable.

Scale adhesion illustrated in FIG. 1 was evaluated by a hot shallowdrawing test in a cylindrical die of φ70 mm and a depth of 20 mm. Aftera steel sheet was heated to a temperature range of 800° C. to 1100° C.at 50° C./s in an electrical heater, and was retained for 0 seconds to120 seconds, energization was stopped, the steel sheet was cooled to650° C. by standing to cool, and hot shallow drawing was performed inthe above described die. Specimens after forming were visually observed,and specimens in which an area where scale was detached accounted for 5%or less were determined as having good (circle) scale adhesion,specimens in which the area where scale was detached accounted for 5 to15% were determined as poor (triangle), and specimens in which the areawhere scale was detached accounted for more than 15% were determined asvery poor (X). The specimens in which the area where scale was detachedaccounted for 5% or less were determined as within the range of thepresent invention.

It is possible to evaluate scale adhesion without particularly limitingthe heating method. For example, conditions of any of a heating furnace,far-infrared rays, near-infrared rays and electrical heating may beadopted. Further, when a steel sheet is heated in a heating furnace,more excellent scale adhesion can be obtained by thinning scale bycontrolling the atmosphere in the heating furnace and restrainingoxidation of the steel sheet.

Note that a shallow drawing test temperature may be in any temperatureregion as long as a steel sheet can be processed, but in general, asteel sheet for hot stamping has high strength and excellent shapefixability by processing in an austenite region and subsequent diehardening. From this, characteristics evaluation was carried out by hotshallow drawing at 650° C. exceeding Ar3.

As an oil coating method, electrostatic oil coating, spray, a rollcoater and the like are generally used, but the oil coating method isnot limited as long as the coating oil amount can be ensured.

Although the kind of oil is not specified, NOX-RUST530F (made by PARKERINDUSTRIES, INC.) or the like is generally used if the oil is mineraloil, for example, and if the coating oil amount satisfies the range ofthe present invention, the kind of oil is not limited.

Although the coating oil amount may be measured by any method as long asthe coating oil amount can be measured, the present inventors measuredthe coating oil amount by the following method. The steel sheet coatedwith rust inhibiting oil was cut into 150 mm square first, andthereafter, a tape was applied so that a 100 mm by 100 mm region isexposed. Subsequently, the weights of the coating oil and the steelsheet to which seal was carried out (including the weight of the tape)were measured in advance. Subsequently, degreasing was performed bywiping off the rust inhibiting oil on the steel sheet surface with clothcontaining acetone, the weight of the degreased steel sheet wasmeasured, the weights before and after degreasing were compared, andthereby the coating oil amount per unit area was calculated. Measurementwas carried out at three spots in each of the steel sheets, and anaverage value of the attached amounts was determined as a coating oilattaching amount of each of the steel sheets.

It is preferable to restrict the S content contained in the rustinhibiting oil to 5 mass % or less. When the present inventorsinvestigated the relationship between the S content in the coating oiland a scale detached area ratio as illustrated in FIG. 2, the presentinventors have found that as the S content in the coating oil becomessmaller, the scale adhesion increases, and especially when the S contentin the coating oil is 5 mass % or less, the scale detached area becomessubstantially 0%. It is conceivable that while the oil contained in therust inhibiting oil is burned and eliminated during heating, S containedas an impurity remains on the steel sheet surface to concentrate intoscale, and thereby deteriorates scale adhesion, although detailedmechanism is unclear. Hence, it is preferable to reduce the content of Scontained in the rust inhibiting oil. The S content is preferably mass %or less, and is more preferably 3 mass % or less. Although analysis of Sin the rust inhibiting oil may be performed by any method as long as Scan be analyzed, the present inventors extracted 5 mL of the rustinhibiting oil which is applied to the steel sheet, and carried outanalysis by fluorescence X-rays (C-ray Fluorescence Sulfur-in-OilAnalyzer SLFA-2800/HORIBA). In measurement, measurement was carried outwith n=3, and an average value thereof was defined as the S content.

The surface roughness of the steel sheet will be described next. Inorder to ensure scale adhesion, the surface roughness of the steel sheetneeds to satisfy Rz>2.5 μm. A result obtained by investigating arelationship between the surface roughness Rz of the steel sheet andscale adhesion is as illustrated in FIG. 1 described above. By providingirregularities on an interface between scale that is generated at thetime of hot stamping thermal treatment and a base iron, theirregularities are formed on the interface between the base iron andscale, and further increase in adhesion is brought about. The effect isgenerally referred to as an anchor effect. In particular, scale that isgenerated at the time of heating in the present steel sheet is thin. Asa result, in the present steel sheet in which the thickness of the scaleis thin, scale having irregularities is formed by receiving an influenceof the base iron surface state. Hence, the surface roughness of thesteel sheet before hot stamping needs to satisfy Rz>2.5 μm. When Rz2.5μm, the surface roughness of the steel sheet is small, and the anchoreffect is insufficient, and thus excellent scale adhesion at the time ofhot stamping cannot be ensured. Although the effect of the excellentscale adhesion of the present invention can be obtained withoutparticularly providing the upper limit, if scale adhesion is excessivelyincreased, it becomes difficult to remove scale in a downstream processsuch as shot blast, for example. Thus, it is preferable to set Rz<8.0μm. It is more preferable to set Rz<7.0 μm. However, even if Rz8.0 μm isset, it is possible to ensure excellent scale adhesion that is theeffect of the present invention. Note that in the steel sheet in whichan Si content is less than 0.50 mass %, even if the surface roughness ofRz>2.5 μm is set, thick Fe scale is formed at the time of heating, andthus even when the irregularities are on the steel sheet surface, theinterface between the base iron and the scale becomes flat by excessiveoxidation. As a result, the irregularities in the interface between thescale and the base iron are eliminated, and the effect of the excellentscale adhesion that is the effect of the present invention is notexhibited.

Although measurement of the surface roughness Rz may be performed by anymethod, the present inventors measured the region of a length of 10 mmwith n=3, with use of a contact surface roughness measuring instrument(SURFCOM2000DX/SD3 made by TOKYO SEIMITSU CO., LTD) with a probe pointangle of 60°, and a point R of 2 μm, and determined the average value asthe surface roughness Rz of each of the steel sheets.

Next, a scale structure of the hot stamp formed body will be described.The steel sheet for hot stamping of the present invention ensures scaleadhesion by control of the irregularities in the interface between thescale and the base iron. Hence, the scale can be scale mainly composedof an Si oxide, Fe₃O₄, Fe₂O₃ and FeO. An Si oxide exists in theinterface between base iron and iron scale (FeO, Fe₂O₃, Fe₂O₃), andthereby controls a thickness of the iron scale. Hence, the scale needsto contain an Si oxide. Since the main object is to control thethickness of the iron oxide, even if the Si oxide is very thin, it issufficient if the Si oxide exists, and even with 1 nm, the Si oxideexhibits the effect.

Composition analysis of the scale of the formed body was carried out byX-ray diffraction by cutting out the sheet from a bottom of thecylindrical portion of a shallow drawn specimen piece. From a peakintensity ratio of the respective oxides, volume ratios of therespective Fe oxides were measured. The Si oxide existed very thinly,and the volume ratio was less than 1%, and thus quantitative evaluationin X-ray diffraction was difficult. However, it is possible to confirmthat an Si oxide exists in the interface between the scale and the baseiron by line analysis of EPMA (Electron Probe Micro Analyzer).

The thickness of the scale is preferably 10 μm or less. When thethickness of the scale is 10 μm or less, scale adhesion is enhancedmore. When the thickness of the scale exceeds 10 μm, the scale tends todetach easily due to a thermal stress that works at the time of coolingat the time of hot stamping. Thereafter, in a scale removing processsuch as shot blast or wet blast, fractures occur among Fe scales, and ascale existing on an outer side detaches. As a result, the scale alsohas a problem of being inferior in scale removability. Hence, thethickness of the scale is preferably 10 μm or less. The thickness of thescale is more preferably 7 μm or less, and is more preferably 5 μm orless. The thickness of the scale is achieved by controlling the coatingoil amount within the predetermined range simultaneously withcontrolling the Si content of the steel sheet within a predeterminedrange. FIG. 5 illustrates a relationship between the coating oil amountand the scale thickness.

In the interface between the base iron and the scale in the hot stampformed body of the present invention, three or more irregularities of0.2 μm to 8.0 μm are present per 100 μm. FIG. 6A shows a photograph ofan interface between a base iron and scale of a formed body excellent inscale adhesion, and FIG. 6B shows a photograph of an interface between abase iron and scale inferior in scale adhesion. Since the irregularitiescontribute to enhancement in scale adhesion at the time of hot stamping,and thus excellent scale adhesion can be ensured by controlling theirregularities within the above described range. Irregularities of lessthan 0.2 μm provide an insufficient anchor effect, and provide inferiorscale adhesion. With irregularities of 8.0 μm or more, scale adhesion isso strong that scale is difficult to remove in the subsequent scaleremoval process, for example, by shot blast or wet blast, and thereforeit is preferable to make the irregularities in the interface betweenscale and the base iron 8.0 μm or less. The irregularities are morepreferably 6.0 μm or less, and more preferably 4.0 μm or less. Note thateven if the irregularities exceed 8.0 μm, excellent scale adhesivenessthat is the effect of the present invention can be ensured.

When the number of irregularities of 0.2 μm to 8.0 μm per 100 μm is lessthan three, an improvement effect of scale adhesion is not sufficient,and thus the number of irregularities per 100 μm is set at three ormore. It is possible to ensure excellent scale adhesion which is theeffect of the present invention without particularly setting an upperlimit of the number of irregularities per 100 μm. Note that theirregularities of the formed body are correlated with the surfaceroughness Rz of the steel sheet as illustrated in FIG. 7, and arecontrollable by setting the steel sheet surface roughness as Rz>2.5 μm.

Next, chemical compositions of the steel sheet and the hot stamp formedbody of the present invention will be described. Note that hereunder %means mass %.

C: 0.100% to 0.600%

C represents an element that is contained to enhance the strength of thesteel sheet. If a C content is less than 0.100%, tensile strength of1180 MPa or more cannot be ensured, and a formed body with high strengthwhich is the object of hot stamp cannot be ensured. When the C contentexceeds 0.600%, weldability and processibility become insufficient, andthus the C content is set at 0.100% to 0.600%. The C content ispreferably 0.100% to 0.550%, and is more preferably 0.150% to 0.500%.However, if the strength of the formed body is not required, excellentscale adhesion can be ensured even if the C content is less than 0.150%.

Si: 0.50% to 3.00%

Si enhances scale adhesion by controlling the scale composition at thetime of hot stamping, and therefore Si is an essential element. If theSi content is less than 0.50%, the thickness of Fe scale cannot becontrolled, and excellent scale adhesion cannot be ensured.Consequently, it is necessary to set the Si content at 0.50% or more.Further, when application to a member which is difficult to form at thetime of hot stamping is considered, it is preferable to increase the Sicontent. Accordingly, the Si content is preferably 0.70% or more, and ismore preferably 0.90% or more. Meanwhile, Si increases an Ae3 point, andthe heating temperature necessary to make martensite a main phase, andthus if the Si is excessively contained, productivity and economicefficiency are reduced. Hence, an upper limit of the Si content is setas 3.00%. The upper limit of the Si content is preferably 2.5%, and theupper limit is more preferably 2.0%. However, it is possible to ensureexcellent scale adhesion excepting productivity and economic efficiency.

Mn: 1.20% to 4.00%

Mn delays ferrite transformation in a cooling process at the time of hotstamping, and makes a hot stamp formed body into a structure having amartensite main phase, and thus it is necessary to contain 1.20% or moreof Mn. If the Mn content is less than 1.20%, martensite cannot be made amain phase, and it is difficult to ensure high strength which is anobject of the hot stamp formed body, and thus a lower limit of the Mncontent is set as 1.20%. However, if the strength of the formed body isnot required, excellent scale adhesion can be ensured even if the Mncontent is less than 1.20%. When the Mn content exceeds 4.00%, theeffect is saturated, embrittlement is caused, and a fracture is causedat the time of casting, cold rolling or hot rolling, and thus an upperlimit of the Mn content is set as 4.00%. The Mn content is preferablywithin a range of 1.50% to 3.50%, and is more preferably within a rangeof 2.00% to 3.00%.

Ti: 0.005% to 0.100%

Ti is an element that combines with N to form TiN, and thereby restrainsB from being a nitride to enhance hardenability. The effect becomesremarkable when a Ti content is 0.005% or more, and thus the Ti contentis set as 0.005% or more. However, when the Ti content exceeds 0.100%, aTi carbide is formed, an amount of C that contributes to strengtheningmartensite is reduced, and reduction in strength is caused, and thus anupper limit of the Ti content is set as 0.100%. The C content ispreferably within a range of 0.005% to 0.080%, and is more preferablywithin a range of 0.005% to 0.060%.

B: 0.0005% to 0.0100%

B enhances hardenability at the time of hot stamping, and contributes tomaking a main phase of martensite. The effect is remarkable when a Bcontent is 0.0005% or more, and thus it is necessary to set the Bcontent at 0.0005% or more. When the B content exceeds 0.0100%, theeffect is saturated, an iron boride is precipitated, and the effect ofhardenability of B is lost, and thus an upper limit of the B content isset at 0.0100%. The B content is preferably within a range of 0.0005% to0.0080%, and is more preferably within a range of 0.0005% to 0.0050%.

P: 0.100% or less

P is an element that segregates in a central portion of a sheetthickness of the steel sheet, and is an element that embrittles a weldedportion. Accordingly, an upper limit of a P content is set at 0.100%. Amore preferable upper limit is 0.050%. The lower the P content, the morepreferable, and although the effect of the present invention isexhibited without particularly setting the lower limit, but it iseconomically disadvantageous to reduce P to less than 0.001% from theviewpoint of productivity and cost of dephosphorization, and thus thelower limit is preferably set at 0.001%.

S: 0.0001% to 0.0100%

S exerts a large influence on scale adhesion, and thus it is necessaryto restrict a content in the steel sheet. Accordingly, an upper limit ofan S content is set at 0.0100%. A lower limit of the S content is set at0.0001% because it is economically disadvantageous from the viewpoint ofproductivity and cost of dephosphorization. The S content is preferablywithin a range of 0.0001% to 0.0070%, and is more preferably within arange of 0.0003% to 0.0050%.

Al: 0.005% to 1.000%

Al acts as a deoxidizer, and thus an Al content is set as 0.005% ormore. When the Al content is less than 0.005%, a sufficientdeoxidization effect cannot be obtained, and a large amount of enclosure(oxide) exist in the steel sheet. These enclosures become startingpoints of destruction at the time of hot stamping, and the causes ofbreakage, and therefore are not preferable. The effect becomesremarkable when the Al content reaches 0.005% or more, and thus it isnecessary to set the Al content at 0.005% or more. When the Al contentexceeds 1.000%, the Ac3 point is increased and a heating temperature atthe time of hot stamping is increased. That is, hot stamp is a techniqueof obtaining a formed body with high strength having a complicated shapeby heating a steel sheet to an austenite single phase region, andsubjecting the steel sheet to hot die press excellent in formability,and rapidly cooling by using a die. As a result, when a large amount ofAl is contained, the Ac3 point is significantly increased, increase inthe heating temperature required for austenite single phase regionheating is caused, and productivity is reduced. Consequently, it isnecessary to set an upper limit of the Al content at 1.000%. The Alcontent is preferably within a range of 0.005% to 0.500%, and is morepreferably within a range of 0.005% to 0.300%.

N: 0.0100% or less

N is an element that forms coarse nitrides and deteriorates bendabilityand hole-expandability. When an N content exceeds 0.0100%, bendabilityand hole-expandability are significantly deteriorated, and thus an upperlimit of the N content is set at 0.0100%. Note that N becomes a cause ofgenerating a blowhole at the time of welding, and thus the smaller the Ncontent is, the more preferable. Accordingly, the N content ispreferably 0.0070 or less, and is more preferably 0.0050% or less.Although it is not necessary to particularly set a lower limit of the Ncontent, manufacturing cost increases significantly when the N contentis reduced to less than 0.0001%, and thus a practical lower limit is0.0001%. From the viewpoint of manufacturing cost, the N content is morepreferably 0.0005% or more.

Note that other unavoidable elements may be contained in extremely smallamounts. For example, 0 forms an oxide and exists as an enclosure.

The steel sheet of the present invention further contains the followingelements in accordance with necessity.

Ni: 0.01% to 2.00%

Cu: 0.01% to 2.00%

Cr: 0.01% to 2.00%

Mo: 0.01% to 2.00%

Ni, Cu, Cr and Mo are elements that contribute to increase in strengthby enhancing hardenability at the time of hot stamping, and making amain phase of martensite. The effect becomes remarkable by containing0.01% or more of each one kind or two or more kinds selected from agroup consisting of Ni, Cu, Cr and Mo, and thus contents of the elementsare preferably 0.01% respectively. When the content of each of theelements exceeds a predetermined amount, weldability, hot workabilityand the like are deteriorated, or the strength of the steel sheet forhot stamping is so high as to be likely to cause a manufacturingtrouble, and thus upper limits of the contents of these elements arepreferably set at 2.00%.

Nb: 0.005 to 0.100%

V: 0.005 to 0.100%

W: 0.005 to 0.100%

Nb, V and W are elements that strengthen fine grains by inhibitinggrowth of austenite at the time of hot stamping, and contribute toincrease in strength and enhancement in tenacity. Hence, one kind or twoor more kinds selected from a group consisting of these elements may becontained. The effect becomes more remarkable when 0.005% or more ofeach of the elements are contained, and thus it is preferable that0.005% or more of each of the elements be contained. Note that when morethan 0.100% of each of these elements is contained, it is not preferablebecause Nb, V and W carbides are formed, an amount of C that contributesto strengthening martensite is reduced, and reduction in strength iscaused. Each of the elements is preferably in a range of 0.005% to0.090%.

A total of one kind or two or more kinds selected from a groupconsisting of REM, Ca, Ce and Mg: 0.0003% to 0.0300%

In the present invention, 0.0003% to 0.0300% of one kind or two or morekinds selected from a group consisting of REM, Ca, Ce and Mg may befurther contained in total.

REM, Ca, Ce and Mg are elements that enhance strength and contribute toimprovement of the material. When the total of one kind or two or morekinds selected from the group consisting of REM, Ca, Ce and Mg is lessthan 0.0003%, a sufficient effect cannot be obtained, and thus it ispreferable to set a lower limit of the total at 0.0003%. When the totalof one kind or two or more kinds selected from the group consisting ofREM, Ca, Ce and Mg exceeds 0.0300%, castability and hot workability arelikely to be deteriorated, and thus it is preferable to set an upperlimit of the total at 0.0300%. Note that REM is an abbreviation of RareEarth Metal, and refers to an element belonging to a lanthanoid system.In the present invention, REM is often added in misch metal, and besidesCe, elements of a lanthanoid system are sometimes contained incombination.

In the present invention, the effect of the present invention becomesapparent even when elements of a lanthanoid system other than La and Ceare contained as unavoidable impurities, and the effect of the presentinvention becomes apparent even when the other elements such as metalsare contained as impurities.

Next, features of microstructures of the steel sheet for hot stampingand hot stamp formed body of the present invention will be described.

Provided that the chemical composition, the surface roughness of thesteel sheet, and the coating oil amount satisfy the ranges of thepresent invention, the effect of the present invention can be exhibitedby any of a pickled hot-rolled steel sheet, a cold-rolled steel sheetobtained by cold-rolling a hot-rolled steel sheet, or a cold-rolledsteel sheet to which annealing is applied after cold rolling.

These steel sheets are heated to an austenite region exceeding 800° C.at the time of hot stamping, and therefore exhibit performance as steelsheets for hot stamping having excellent scale adhesion that is theeffect of the present invention without particularly limiting themicrostructure. However, when mechanical cutting of the steel sheets andcold punching are carried out prior to hot stamping, the strength of thesteel sheets is preferably as low as possible in order to reduce wearand tear of dies, cutting edges of cutters, or punching dies.Consequently, the microstructure of the steel sheet for hot stamping ispreferably ferrite and pearlite structures, or a bainite structure and astructure obtained by tempering martensite. However, if wear and tear ofa punch and dies at the time of mechanical cutting and cold punching donot become a problem, it is possible to ensure excellent scale adhesionwhich is the effect of the present invention, even if one kind or two ormore kinds of retained austenite, martensite in a hardened state, andbainite are contained. Further, in order to reduce the strength of thesteel sheet, thermal treatment in a box type annealing furnace or acontinuous annealing facility may be carried out. Alternatively, evenwhen cold rolling is carried out after the above softening treatment,and the sheet thickness is controlled to a predetermined sheetthickness, excellent scale adhesion which is the effect of the presentinvention is ensured.

When formed body strength after hot stamping is enhanced, and highcomponent strength is obtained, the microstructure of the formed bodypreferably has a martensite main phase. In particular, in order toensure tensile strength of 1180 MPa or more, a volume ratio ofmartensite that is a main phase is preferably made 60% or more.Martensite may be subjected to tempering after hot stamping, and madetempered martensite. As the structure other than martensite, bainite,ferrite, pearlite, cementite and retained austenite may be contained.Further, even if the martensite volume rate is less than 60%, it ispossible to ensure the excellent scale adhesion of the presentinvention.

The following methods are used in identification of the microstructures(tempered martensite, martensite, bainite, ferrite, pearlite, retainedaustenite and a remaining structure) composing the steel sheetstructure, confirmation of existence positions, and measurement of arearatios. For example, it is possible to corrode a section in a steelsheet rolling direction or a section in a direction perpendicular to therolling direction with a nital reagent and the reagent disclosed inJapanese Laid-open Patent Publication No. 59-219473, and observe thestructure with a 1000 to 100000-power scanning electron microscope (SEM:Scanning Electron Microscope) and transmission electron microscope (TEM:Transmission Electron Microscope). The present inventors determined thesheet thickness section parallel with the rolling direction of the steelsheet as an observation surface, extracted a specimen, polished theobservation surface, performed nital etching, observed a range ofthickness of ⅛ to ⅜ with ¼ of the sheet thickness as a center with afield emission scanning electron microscope (FE-SEM: Field EmissionScanning Electron Microscope), measured an area fraction, and the areafraction was taken as a volume fraction. As for the volume fraction ofthe retained austenite, the volume fraction was measured by performingX-ray diffraction with the surface which was parallel with the sheetsurface of the parent steel sheet and had a ¼ of thickness, used as theobservation surface.

Next, a method for producing the steel sheet for hot stamping of thepresent invention will be described.

Although the other operation conditions are based on a usual method, thefollowing conditions are preferable in terms of productivity.

In order to produce the steel sheet in the present invention, a slabhaving the same component composition as the component composition ofthe aforementioned steel sheet is cast first. As the slab provided forhot rolling, a continuously cast slab, the slab produced by a thin slabcaster or the like can be used. The method for manufacturing the steelsheet of the present invention is adapted to a process like continuouscasting-direct rolling (CC-DR) that performs hot rolling immediatelyafter casting.

-   Slab heating temperature: 1100° C. or higher-   Hot-rolling completion temperature: Ar3 transformation point or    higher-   Coiling temperature: 700° C. or lower-   Cold rolling ratio: 30 to 70%

The slab heating temperature is preferably set at 1100° C. or higher.The slab heating temperature in a temperature region of lower than 1100°C. causes reduction in the finishing rolling temperature, and thusstrength at the time of finishing rolling tends to be high. As a result,there is the possibility that rolling becomes difficult, a poor shape ofthe steel sheet after rolling is caused, and thus the slab heatingtemperature is preferably set at 1100° C. or higher.

The finishing rolling temperature is preferably set at the Ar3transformation point or higher. When the finishing rolling temperaturebecomes lower than the Ar3 transformation point, a rolling load becomeshigh, and there is the possibility that rolling becomes difficult, and apoor shape of the steel sheet after rolling is caused, and thus a lowerlimit of the finishing rolling temperature is preferably set at the Ar3transformation point. An upper limit of the finishing rollingtemperature does not have to be particularly set, but if the finishingrolling temperature is set to be excessively high, the slab heatingtemperature has to be made excessively high in order to ensure thetemperature, and thus the upper limit of the finishing rollingtemperature is preferably 1100° C.

The coiling temperature is preferably set at 700° C. or lower. When thecoiling temperature exceeds 700° C., the thickness of the oxides formedon the steel sheet surface is excessively increased, and the picklingproperty is deteriorated, and thus the coiling temperature higher than700° C. is not preferable. When cold rolling is performed thereafter, alower limit of the coiling temperature is preferably set at 400° C. Whenthe coiling temperature is lower than 400° C., the strength of thehot-rolled steel sheet extremely increases, and a sheet fracture and apoor shape at the time of cold rolling are easily caused, and thus thelower limit of the coiling temperature is preferably set at 400° C.However, if the hot-rolled steel sheet which is coiled is intended to besoftened by heating the coiled hot-rolled steel sheet in the box typeannealing furnace or the continuous annealing facility, the steel sheetmay be coiled at a low temperature of lower than 400° C. Note that atthe time of hot-rolling, rough-rolled sheets may be bonded to oneanother and finishing rolling may be continuously performed. Further,the rough-rolled sheet may be coiled temporarily.

Next, pickling is applied to the hot-rolled steel sheet which isproduced in this way for 30 seconds or more in an aqueous solution withan temperature of 80° C. to 100° C. in which a concentration of acid is3 mass % to 20 mass % and an inhibitor is included. In the presentinvention, pickling under the present conditions is extremely important,and in order to control the surface roughness Rz of the steel sheet tomore than 2.5 μm, pickling under the above described conditions isnecessary. Note that an aqueous solution of a hydrochloric acid, asulfuric acid or the like as an acid is generally used, and an aquaregia or the like may be used.

The temperature of the aqueous solution is set at 80° C. to lower than100° C., because with a temperature lower than 80° C., a reaction rateis low, and it takes a long time to bring the surface roughness of thehot-rolled steel sheet into a proper range. Meanwhile, heating at atemperature of 100° C. or higher is dangerous and is not preferablebecause the solution boils and splashes although the reaction ofpickling has no problem.

Further, the reason why the concentration of the acid is set at 3 mass %to 20 mass % is to control the surface roughness Rz of the hot-rolledsteel sheet within the proper range. When the concentration of the acidis less than 3 mass %, it takes a long time to control theirregularities on the surface by pickling. When the concentration of theacid exceeds 20 mass %, a pickling tank is damaged significantly andfacility management becomes difficult, and thus it is not preferable. Apreferable range of the concentration of the acid is a range of 5 mass %to 15 mass %.

Further, the reason why the pickling time period is set at 30 seconds ormore is to stably give predetermined irregularities (irregularities ofRz>2.5 μm) to the steel sheet surface by pickling. When the picklingtank is divided into a plurality of tanks, if a pickling time period ofsome of the pickling tanks or a total pickling time period satisfies theabove described conditions, the surface roughness Rz of the hot-rolledsteel sheet can be brought into the range of the present invention, evenif concentrations or temperatures of the individual pickling tanksdiffer from one another. Further, pickling may be carried out by beingdivided into a plurality of times. Note that in the experiment by thepresent inventors, a hydrochloric acid including an inhibitor was used,but the effect of the present invention can be obtained by using anotheracid such as hydrochloric acid using no inhibitor, a sulfuric acid, anda nitric acid, or a composite of these acids, as long as the surfaceroughness Rz can be controlled by pickling.

Further, the irregularities formed by pickling of the hot-rolled steelsheet also remain even after temper rolling, cold rolling or annealingis carried out, and thus it is extremely important to control thepickling conditions, and give irregularities to the sheet surface afterpickling. Consequently, temper rolling may be carried out to thehot-rolled steel sheet after pickling.

Further, even with a cold-rolled steel sheet to which only cold rollingis performed, or a cold-rolled steel sheet thermally treated in acontinuous annealing facility or a box type annealing furnace after coldrolling, irregularities are formed on the surface by performing picklingbefore cold rolling, and the predetermined effect can be obtained. Notethat cold rolling is preferably performed with roll roughness Rz forcold rolling within a range of 1.0 μm to 20.0 μm, and the cold rollingroll also includes temper rolling roll.

Cold rolling is applied to the hot-rolled steel sheet pickled under theconditions as above at a draft of 30% to 80%, and the steel sheet may bepassed through a continuous annealing facility. When the draft is lessthan 30%, it becomes difficult to keep the shape of the steel sheetflat, and ductility of the finished product deteriorates, and thus alower limit of the draft is preferably set at 30%. When the draftexceeds 80%, a rolling load becomes excessively large, and cold rollingbecomes difficult, and thus an upper limit of the draft is preferablyset at 80%. The draft is more preferably 40% to 70%. The effect of thepresent invention becomes apparent even without particularly specifyingthe number of times of rolling pass and the draft of each pass, and thusit is not necessary to specify the number of times of rolling pass, andthe draft at each pass.

Thereafter, the cold-rolled steel sheet may be passed through thecontinuous annealing line. An object of the treatment is to soften thesteel sheet which is highly strengthened by cold-rolling, and thus anyconditions may be adopted as long as the condition is such that thesteel sheet is softened. For example, when the annealing temperature isin a range of 550° C. to 750° C., dislocation introduced at the time ofcold rolling is released by recovery, recrystalization, or phasetransformation, and thus annealing is preferably performed in thistemperature region.

By performing annealing by a box type furnace for the similar purpose,the steel sheet for hot stamping excellent in scale adhesion of thepresent invention can be obtained.

Thereafter, oil coating is carried out. As an oil coating method,electrostatic oiling, spray, a roll coater and the like are generallyused, and as long as a coating oil amount in a range of 50 mg/m² to 1500mg/m² can be ensured, the method is not limited. In the presentinvention, coating of a predetermined amount of oil was carried out byan electrostatic oiling machine. Further, as long as the coating oilamount in the range of 50 mg/m² to 1500 mg/m² can be ensured, a rustinhibitor in an amount equal to or larger than the coating oil amountmay be applied, and degreasing may be performed.

The excellent scale adhesion that is the effect of the present inventionand a rust inhibition property can be made compatible withoutparticularly limiting the hot stamping conditions. For example, byproducing by the production method shown as follows, compatibility ofexcellent performance of the tensile strength of 1180 MPa or more andproductivity is achieved. At the time of performing hot stamping,heating is preferably performed to a temperature region of 800° C. to1100° C. at a heating rate of 2° C./second or more. By heating at a rateof 2° C./second or more, scale generation at the time of heating can berestrained, and the effect of improvement in scale adhesion is provided.The heating rate is preferably 5° C./second or more, and is morepreferably 10° C./second or more. Further, increase of the heating rateis also effective for the purpose of enhancing productivity.

The annealing temperature at the time of performing hot stamping ispreferably within the range of 800° C. to 1100° C. By performingannealing in this temperature region, it is possible to make thestructure into an austenite single phase structure, and the structurecan be made into a structure having martensite as a main phase bycooling that is performed subsequently. When the annealing temperatureat this time is lower than 800° C., the structure at the time ofannealing is made into a ferrite and austenite structures, the ferritegrows in the cooling process, the ferrite volume ratio exceeds 10%, andthe tensile strength of the hot stamp formed body becomes lower than1180 MPa. Consequently, a lower limit of the annealing temperature ispreferably set at 800° C. When the annealing temperature exceeds 1100°C., not only the effect is saturated, but also the scale thickness issignificantly increased, and there arises the fear that scale adhesionis reduced. Consequently, it is preferable to perform annealing at 1100°C. or lower. The annealing temperature is more preferably in a range of830° C. to 1050° C.

After heating, retention may be performed in the temperature region of800° C. to 1100° C. When retention is carried out at a high temperature,melting of carbides included in the steel sheet is possible, andcontribution is made to increase in the strength of the steel sheet andenhancement in hardenability. Retention includes residence, heatingremoval and cooling removal in the present temperature region. Since theobject is to melt the carbides, the object is achieved as long as theresidence time period in the present temperature region is ensured.Although the limitation on the retention time period is not particularlyprovided, 1000 seconds is preferably set as an upper limit, because whenthe retention time period is 1000 seconds or more, the scale thicknessbecomes excessively large, and scale adhesion is deteriorated.

Thereafter, a temperature of 800° C. to 700° C. is preferably reduced atan average cooling rate of 5° C./second or more. Here, 700° C. is a diecooling start temperature, and the reason why the temperature of 800° C.to 700° C. is reduced at 5° C./second or more is to avoid ferritetransformation, bainite transformation and pearlite transformation, andmake the structure into a martensite main phase. When the cooling rateis less than 5° C./second, these soft structures are formed, and it isdifficult to ensure the tensile strength of 1180 MPa or more. Meanwhile,the effect of the present invention is exhibited without particularlysetting the upper limit of the cooling rate. The reason why thetemperature range which is reduced at 5° C./second or more is set from800° C. to 700° C. is that in this temperature range, the structure offerrite or the like that causes reduction in strength is likely to beformed. Cooling at this time is not limited to continuous cooling, andeven when retention and heating in the temperature region are performed,the effect of the present invention is exhibited as long as the averagecooling rate is 5° C./second or more. The effect of the presentinvention can be exhibited without particularly limiting the coolingmethod. That is, the effect of the present invention can be exhibited byeither one of cooling using a die or die cooling using water cooling incombination.

EXAMPLES

Next, examples of the present invention will be described, andconditions in the examples are only one example of the conditionsadopted to confirm implementability and the effect of the presentinvention, and the present invention is not limited to the one conditionexample. The present invention can adopt various conditions as long asthe conditions achieve the object of the present invention withoutdeparting from the gist of the present invention.

First, slabs of the component compositions of A to S and a to n shown inTable 1 were cast, and after the slabs were temporarily cooled to a roomtemperature, heating was carried out for 220 minutes in a heatingfurnace with a furnace temperature=1230° C., hot rolling was carried outwith the finishing rolling temperature=920° C. to 960° C., and coilingwas carried out under the temperature conditions shown in Table 2.

TABLE 1 Chemical component (mass %) C Si Mn P S Ti B N Al Others A 0.2111.04 2.29 0.011 0.0009 0.025 0.0028 0.0023 0.023 — B 0.207 0.67 2.090.009 0.0012 0.023 0.0014 0.0027 0.019 — C 0.189 1.83 2.55 0.007 0.00160.028 0.0029 0.0026 0.035 — D 0.207 1.21 1.44 0.012 0.0018 0.035 0.00090.0031 0.056 Cr = 0.68 E 0.208 1.19 1.82 0.013 0.0022 0.045 0.00220.0024 0.045 Mo = 0.13 F 0.221 1.11 1.74 0.008 0.0019 0.029 0.00230.0022 0.024 Ni = 0.44, Cu = 0.12 G 0.203 1.05 2.27 0.009 0.0017 0.0240.0020 0.0026 0.029 Nb = 0.068 H 0.219 0.98 2.35 0.010 0.0033 0.0210.0024 0.0037 0.018 V = 0.054 I 0.228 1.24 2.19 0.011 0.0027 0.0220.0021 0.0024 0.033 W = 0.033 J 0.218 1.24 2.31 0.015 0.0045 0.0260.0034 0.0034 0.027 REM = 0.0046 K 0.234 1.05 2.37 0.016 0.0039 0.0240.0025 0.0021 0.025 Ca = 0.0033 L 0.219 1.03 2.19 0.009 0.0048 0.0250.0021 0.0038 0.011 Ce = 0.0029 M 0.246 1.11 2.27 0.013 0.0052 0.0190.0021 0.0029 0.007 Mg = 0.0019 N 0.309 1.09 2.19 0.008 0.0024 0.0160.0026 0.0019 0.022 — O 0.314 0.78 2.11 0.013 0.0028 0.025 0.0018 0.00240.030 — P 0.311 1.32 1.87 0.011 0.0030 0.028 0.0020 0.0023 0.035 Cr =0.18 Q 0.356 1.24 2.09 0.015 0.0034 0.022 0.0034 0.0025 0.029 — R 0.3491.06 1.43 0.016 0.0019 0.021 0.0032 0.0022 0.027 Cr = 0.46 S 0.412 0.992.22 0.007 0.0014 0.028 0.0023 0.0020 0.024 — a 0.097 0.98 2.03 0.0220.0022 0.033 0.0028 0.0023 0.021 — b 0.698 1.49 1.68 0.007 0.0006 0.0240.0026 0.0029 0.089 — c 0.203 0.34 2.11 0.015 0.0027 0.029 0.0026 0.00290.019 — d 0.194 3.25 2.09 0.009 0.0018 0.021 0.0031 0.0025 0.023 — e0.211 1.03 1.12 0.014 0.0016 0.026 0.0018 0.0021 0.026 — f 0.199 1.237.89 0.024 0.0039 0.025 0.0049 0.0042 0.038 — g 0.205 0.78 2.31 0.0090.0148 0.021 0.0012 0.0031 0.022 — h 0.209 1.16 2.31 0.007 0.0013 —0.0018 0.0034 0.031 — i 0.184 1.08 1.42 0.006 0.0037 0.139 0.0034 0.00370.022 — j 0.210 1.05 1.89 0.016 0.0035 0.022 — 0.0028 0.027 — k 0.2010.98 1.45 0.011 0.0055 0.027 0.1180 0.0033 0.024 — l 0.198 1.21 1.520.008 0.0042 0.033 0.0027 0.0191 0.046 — m 0.205 0.87 2.42 0.011 0.00570.021 0.0016 0.0082 0.001 — n 0.213 0.94 1.98 0.012 0.0019 0.023 0.00240.0024 1.285 — The underlined part means being outside the range of thepresent invention. “—” means that each element is not added.

TABLE 2 Coiling Acid Acid Pickling Coating oil Steel Steel temperatureconcentration temperature time period Kind of amount S content numbergrade*1 (° C.) (%) (° C.) (s) coating oil (mg/m²) (mass %) Remarks A1 FH580  8 83 160 NOX503F — 0 Comparative steel A2 FH 600  6 87 200 NOX503F 60 0 Present invention steel A3 FH 590  8 85 160 NOX503F  140 1 Presentinvention steel A4 FH 680  7 90 680 NOX503F  270 1 Present inventionsteel A5 FH 590  9 89 160 NOX503F  490 1 Present invention steel A6 FH600  9 86 160 NOX503F  780 1 Present invention steel A7 FH 580  8 84 240NOX503F 1020 1 Present invention steel A8 FH 550  9 86  40 NOX503F 14805 Present invention steel A9 FH 510  6 82  18 NOX503F 1000 2Comparative steel A10 FH 520  7 85  24 NOX503F  970 1 Comparative steelA11 FH 510  6 85  28 NOX503F  820 1 Comparative steel A12 FH 620 10 94180 NOX503F 1790 6 Comparative steel A13 FH 560  8 89  65 NOX503F 2050 6Comparative steel A14 FH 600  9 85 200 NOX503F 3720 7 Comparative steelA15 FH 570  8 87 240 NOX504F 4890 9 Comparative steel B1 HR 580 12 85230 NOX503F  490 1 Present invention steel C1 CR 590  6 86 160 NOX503F 420 1 Present invention steel D1 FH 560  8 85 100 NOX503F  550 1Present invention steel E1 FH 560  7 83 100 NOX503F 1030 2 Presentinvention steel F1 FH 570  6 88 140 NOX503F 1200 3 Present inventionsteel G1 FH 610  8 93 200 NOX503F  820 1 Present invention steel H1 FH600 10 89 130 NOX503F  670 1 Present invention steel I1 FH 580  8 86 240NOX503F  980 0 Present invention steel J1 FH 550  9 90  80 NOX503F 11802 Present invention steel K1 FH 570  8 84 160 NOX503F  630 1 Presentinvention steel L1 FH 590  9 88 220 NOX503F  940 0 Present inventionsteel M1 FH 600  6 90 200 NOX503F  430 1 Present invention steel N1 FH590  8 83 200 NOX503F  570 1 Present invention steel N2 FH 560  9 89  80NOX503F  690 1 Present invention steel N3 FH 550 11 92  70 NOX503F  7001 Present invention steel N4 FH 600 10 94 120 NOX503F  800 1 Presentinvention steel N5 FH 520  7 82  18 NOX503F  760 1 Comparative steel N6FH 530  7 83  26 NOX503F  80 1 Comparative steel N7 FH 590  8 86 210NOX503F — 0 Comparative steel N8 FH 570  9 87 190 NOX504F 3560 6Comparative steel N9 FH 600  9 92 200 NOX503F 4820 8 Comparative steelN10 FH 590  8 88 240 NOX503F 6090 7 Comparative steel O1 HR 590  8 94240 NOX503F 1220 2 Present invention steel P1 CR 580  7 86 200 NOX503F 890 1 Present invention steel Q1 FH 580  9 86 190 NOX503F  800 1Present invention steel R1 HR 590  8 83 200 NOX503F 1370 3 Presentinvention steel S1 FH 560  9 89 200 NOX503F  680 1 Present inventionsteel a1 FH 590  8 85 240 NOX503F  590 1 Comparative steel b1 —*2 —*2—*2 —*2 —*2 —*2 —*2 —*2 Comparative steel c1 FH 560  8 84 240 NOX503F1260 2 Comparative steel d1 FH 480  7 86  18 NOX503F 2450 2Comparative steel e1 FH 570  9 90 270 NOX503F  990 1 Comparative steelf1 —*2 —*2 —*2 —*2 —*2 —*2 —*2 —*2 Comparative steel g1 FH 580  9 88 210NOX503F 1210 1 Comparative steel h1 FH 560  8 92 180 NOX503F 1040 0Comparative steel i1 FH 590  7 89 220 NOX503F 1300 1 Comparative steelj1 FH 570  8 88 200 NOX503F 1230 2 Comparative steel k1 FH 640  8 85 190NOX503F  840 1 Comparative steel l1 FH 610  9 82  80 NOX503F  900 2Comparative steel m1 FH 560  9 93 280 NOX503F 1000 1 Comparative steeln1 FH 560  9 86 180 NOX503F  570 1 Comparative steel *1 means that FH:left as cold rolled, HR: hot-rolled steel sheet, and CR: cold-rolledsteel sheet annealed after cold rolling. *2 means that Mn is excessvelyhigh, many fractures occur in casting and hot rolling time, and nohot-rolled steel sheet was able to be produced.

The finished sheet thickness of the hot-rolled steel sheet provided forhot stamping as the hot-rolled steel sheet was made 1.6 mm. The sheetthickness of the hot-rolled steel sheet provided for cold rolling wasmade 3.2 mm. When pickling was carried out under the conditions in Table2 thereafter, and cold rolling was performed, the sheet thickness wasmade 50% (3.2 mm→1.6 mm). Thereafter, annealing was performed for someof the steel sheets in a continuous annealing facility, and the steelsheets were made into cold-rolled steel sheets. Thereafter, by usingNOX-RUST503F (made by PARKER INDUSTRIES, INC.), NOX503F (made by PARKERINDUSTRIES, INC.) was applied to the hot-rolled steel sheets and thecold-rolled steel sheets by an electrostatic oiling machine, in a rangeof no coating oil to 6090 mg/m².

Thereafter, the steel sheets were cut into a predetermined size, afterwhich, electrical heating was performed to 900° C. at 50° C./second,retention for seconds at 900° C. was carried out, thereafter, standingto cool for 10 seconds was performed, and hardening was performed in theabove described hot shallow drawing dies at a temperature of 650° C. orhigher. Visual observation of the obtained hot stamp formed bodies wasperformed, and the steel sheets without detachment of scale weredetermined as the steel sheets excellent in scale adhesion.

Concerning the rust inhibition property, retention for 30 days wascarried out at a room temperature, and the steel sheets with no rustgenerated on the steel sheet surfaces were defined as the steel sheetsexcellent in rust inhibition property. In combination, with use of flatsheet test pieces, hot stamping was performed under the aforementionedconditions, and tensile characteristics were evaluated. The evaluationresult is shown in Table 3.

TABLE 3 Presence or TS of Scale Irregularities Scale absence of formedSteel Steel Rz thickness in scale/base detached Scale rust body numbergrade*1 (μm) (μm) iron interface area (%) adhesion generation (MPa)Remarks A1 FH 3.7  4  6  0 ◯ Presence 1555 Comparative steel A2 FH 3.6 5  5  0 ◯ Absence 1562 Present inventon steel A3 FH 4.0  4  8  0 ◯Absence 1560 Present inventon steel A4 FH 4.4  5  9  0 ◯ Absence 1559Present inventon steel A5 FH 4.2  4  6  0 ◯ Absence 1555 Presentinventon steel A6 FH 4.5  6  8  0 ◯ Absence 1550 Present inventon steelA7 FH 3.8  6  7  0 ◯ Absence 1557 Present inventon steel A8 FH 2.6  5  4 3 ◯ Absence 1562 Present inventon steel A9 FH 2.2  2  0 11 Δ Absence1562 Comparative steel A10 FH 1.9  3  1 16 X Absence 1554Comparative steel A11 FH 2.4  2  2  8 Δ Absence 1564 Comparative steelA12 FH 4.8 12  7 14 Δ Absence 1549 Comparative steel A13 FH 2.6 14  4 26X Absence 1556 Comparative steel A14 FH 3.9 15  8 44 X Absence 1560Comparative steel A15 FH 3.6 18  7 68 X Absence 1567 Comparative steelB1 HR 5.9  4 13  0 ◯ Absence 1549 Present inventon steel C1 CR 4.1  4  6 0 ◯ Absence 1483 Present inventon steel D1 FH 3.7  3  5  0 ◯ Absence1529 Present inventon steel E1 FH 3.8  3  8  0 ◯ Absence 1550 Presentinventon steel F1 FH 3.7  7  7  0 ◯ Absence 1625 Present inventon steelG1 FH 4.5  4  8  0 ◯ Absence 1572 Present inventon steel H1 FH 4.6  4  6 0 ◯ Absence 1645 Present inventon steel I1 FH 4.4  5  6  0 ◯ Absence1687 Present inventon steel J1 FH 3.8  5  8  0 ◯ Absence 1639 Presentinventon steel K1 FH 4.0  3  7  0 ◯ Absence 1752 Present inventon steelL1 FH 4.5  4  8  0 ◯ Absence 1624 Present inventon steel M1 FH 4.3  4  7 0 ◯ Absence 1715 Present inventon steel N1 FH 4.4  4  7  0 ◯ Absence1834 Present inventon steel N2 FH 3.9  3  6  0 ◯ Absence 1828 Presentinventon steel N3 FH 3.2  3  5  0 ◯ Absence 1833 Present inventon steelN4 FH 4.5  4 10  0 ◯ Absence 1829 Present inventon steel N5 FH 2.3  2  0 9 Δ Absence 1830 Comparative steel N6 FH 1.8  3  1 13 Δ Absence 1826Comparative steel N7 FH 3.9  4  8  0 ◯ Presence 1834 Comparative steelN8 FH 4.6 13  8 39 X Absence 1823 Comparative steel N9 FH 4.3 13  7 47 XAbsence 1835 Comparative steel N10 FH 4.5 21  8 45 X Absence 1830Comparative steel O1 HR 5.3  4 13  0 ◯ Absence 1854 Present inventonsteel P1 CR 4.4  4  8  0 ◯ Absence 1847 Present inventon steel Q1 FH 4.7 4  9  0 ◯ Absence 2108 Present inventon steel R1 HR 6.0  4 12  0 ◯Absence 2138 Present inventon steel S1 FH 3.9  3  7  0 ◯ Absence 2505Present inventon steel a1 FH 4.3  4  8  0 ◯ Absence 1054Comparative steel b1 FH —*2 —*2 —*2 —*2 —*2 —*2 —*2 Comparative steel c1FH 2.4 16  0 89 X Absence 1483 Comparative steel d1 FH 1.8  1  0 84 XAbsence 1598 Comparative steel e1 FH 3.8  4  9  0 ◯ Absence  987Comparative steel f1 FH —*2 —*2 —*2 —*2 —*2 —*2 —*2 Comparative steel g1FH 4.3  5  9 92 X Absence 1604 Comparative steel h1 FH 4.1  4  8  0 ◯Absence 1156 Comparative steel i1 FH 4.0  5  7  0 ◯ Absence 1095Comparative steel j1 FH 4.6  4  7  0 ◯ Absence 1023 Comparative steel k1FH 4.4  7 11  0 ◯ Absence 1154 Comparative steel l1 FH 4.1  5  9  0 ◯Absence 1072 Comparative steel m1 FH 3.9  4  5  0 ◯ Absenco —*3Comparative steel n1 FH 4.5  4  7  0 ◯ Absence 1008 Comparative steel *1means that FH: left as cold rolled, HR: hot-rolled steel sheet, and CR:cold-rolled steel sheet annealed after cold rolling. *2 means that Mnwas excessively high, many fractures occurred in casting and hot rollingtime, and no hot-rolled steel sheet was able to be produced. *3 meansthat at the time of hot stamping, a fracture with the enclosure as thestarting point occurred, and the tensile test was not be able to becarried out with the formed

As for the tensile characteristics, the tensile test pieces which werein conformity with JIS Z 2201 were extracted, the tensile test wasperformed in conformity with JIS Z 2241, and the maximum tensilestrength was measured. The formed bodies having the maximum tensilestrength of 1180 MPa or more were determined as the formed bodies of thepresent invention.

Composition analyses of the scales of the formed bodies were carried outby X-ray diffraction by cutting out sheets from the bottoms of thecylindrical portions of the shallow drawing test pieces. From the peakstrength ratios of the respective oxides, the volume ratios of therespective Fe oxides were measured. The Si oxides were present verythinly and the volume ratio was less than 1%, and thus quantitativeevaluation by X-ray diffraction was difficult. However, it could beconfirmed that the Si oxides were present in the interface between thescale and the base iron by the line analysis of EPMA.

As for evaluation of the irregularities in the interfaces of the scalesand the base irons formed in the formed bodies, embedded polishing wascarried out for the steel sheets cut out from the above describedposition, and thereafter, SEM observation was performed by power of 3000from the section perpendicular to the rolling direction. Five visualfields were observed in each of the test pieces, and the number densityof the irregularities in the range of 0.2 μm to 1.0 μm per length of 100μm was measured.

The formed bodies satisfying the conditions of the present inventionwere able to make excellent rust inhibition properties and excellentscale adhesion compatible. The formed bodies that do not satisfy theconditions of the invention were inferior in scale adhesion, or inferiorin corrosion resistance.

INDUSTRIAL APPLICABILITY

According to the present invention, the steel sheet excellent in scaleadhesion at the time of hot stamping can be provided, the problems ofwear and tear of the die at the time of hot stamping, plating adhesionto the die, and indentation flaws accompanying it can be solved, andthus the present invention can bring about significant enhancement inproductivity, and has an industrially large value.

1. A steel sheet for hot stamping, comprising a composition containing:in mass %, C: 0.100% to 0.600%; Si: 0.50% to 3.00%; Mn: 1.20% to 4.00%;Ti: 0.005% to 0.100%; B: 0.0005% to 0.0100%; P: 0.100% or less; S:0.0001% to 0.0100%; Al: 0.005% to 1.000%; N: 0.0100% or less; Ni: 0% to2.00%; Cu: 0% to 2.00%; Cr: 0% to 2.00%; Mo: 0% to 2.00%; Nb: 0% to0.100%; V: 0% to 0.100%; W: 0% to 0.100%, and a total of one kind or twoor more kinds selected from a group consisting of REM, Ca, Ce and Mg: 0%to 0.0300%, with a balance being Fe and impurities, wherein surfaceroughness of the steel sheet satisfies Rz>2.5 μm, and coating oil in anamount of 50 mg/m² to 1500 mg/m² is applied onto a surface.
 2. The steelsheet for hot stamping according to claim 1, wherein an amount of Scontained in the coating oil which is applied onto the steel sheet is 5%or less in mass %.
 3. The steel sheet for hot stamping according toclaim 1, wherein the composition of the steel sheet contains, in mass %,one kind or two or more kinds selected from a group consisting of Ni:0.01% to 2.00%, Cu: 0.01% to 2.00%, Cr: 0.01% to 2.00%, Mo: 0.01% to2.00%, Nb: 0.005% to 0.100%, V: 0.005% to 0.100%, and W: 0.005% to0.100%.
 4. The steel sheet for hot stamping according to claim 1,wherein the composition of the steel sheet contains, in mass %, a totalof 0.0003% to 0.0300% of one kind or two or more kinds selected from thegroup consisting of REM, Ca, Ce and Mg.
 5. A method for producing asteel sheet for hot stamping, comprising: a step of casting a slabcontaining, in mass %, C: 0.100% to 0.600%; Si: 0.50% to 3.00%; Mn:1.20% to 4.00%; Ti: 0.005% to 0.100%; B: 0.0005% to 0.0100%; P: 0.100%or less; S: 0.0001% to 0.0100%; Al: 0.005% to 1.000%; N: 0.0100% orless; Ni: 0% to 2.00%; Cu: 0% to 2.00%; Cr: 0% to 2.00%; Mo: 0% to2.00%; Nb: 0% to 0.100%; V: 0% to 0.100%; W: 0% to 0.100%, and a totalof one kind or two or more kinds selected from a group consisting ofREM, Ca, Ce and Mg: 0% to 0.0300%, with a balance being Fe andimpurities, and hot rolling the slab directly or by allowing the slab tocool and heating the slab to obtain a hot-rolled steel sheet; a step ofpickling the hot-rolled steel sheet for 30 seconds or more in an aqueoussolution having a temperature of 80° C. to lower than 100° C. andincluding an inhibitor with a concentration of an acid being 3 mass % to20 mass %; and a step of applying a rust inhibiting oil to the steelsheet after carrying out the pickling, wherein a rust inhibiting oilremaining amount on a steel sheet surface is limited to 50 mg/m² to 1500mg/m².
 6. The method for producing a steel sheet for hot stampingaccording to claim 5, wherein the rust inhibiting oil is applied to thehot-rolled steel sheet which has been pickled.
 7. The method forproducing a steel sheet for hot stamping according to claim 5, furthercomprising: a step of cold rolling the hot-rolled steel sheet which hasbeen pickled to obtain a cold-rolled steel sheet, wherein the rustinhibiting oil is applied to the cold-rolled steel sheet.
 8. The methodfor producing a steel sheet for hot stamping according to claim 5,further comprising: a step of cold rolling the hot-rolled steel sheetwhich has been pickled, and further performing thermal treatment in acontinuous annealing facility or a box type annealing furnace to obtaina cold-rolled steel sheet, wherein the rust inhibiting oil is applied tothe cold-rolled steel sheet.
 9. The method for producing a steel sheetfor hot stamping according to claim 5, wherein an amount of S in therust inhibiting oil that is applied to the steel sheet is 5% or less inmass %.
 10. The method for producing a steel sheet for hot stampingaccording to claim 5, wherein a composition of the slab contains, inmass %, one kind or two or more kinds selected from a group consistingof Ni: 0.01% to 2.00%, Cu: 0.01% to 2.00%, Cr: 0.01% to 2.00%, Mo: 0.01%to 2.00%, Nb: 0.005% to 0.100%, V: 0.005% to 0.100%, and W: 0.005% to0.100%.
 11. The method for producing a steel sheet for hot stampingaccording to claim 5, wherein a composition of the slab contains, inmass %, a:total of 0.0003% to 0.0300% of one kind or two or more kindsselected from the group consisting of REM, Ca, Ce and Mg.
 12. A hotstamp formed body, comprising a composition containing: in mass %, C:0.100% to 0.600%; Si: 0.50% to 3.00%; Mn: 1.20% to 4.00%; Ti: 0.005% to0.100%; B: 0.0005% to 0.0100%; P: 0.100% or less; S: 0.0001% to 0.0100%;Al: 0.005% to 1.000%; N: 0.0100% or less; Ni: 0% to 2.00%; Cu: 0% to2.00%; Cr: 0% to 2.00%; Mo: 0% to 2.00%; Nb: 0% to 0.100%; V: 0% to0.100%; W: 0% to 0.100%, and a total of one kind or two or more kindsselected from a group consisting of REM, Ca, Ce and Mg: 0% to 0.0300%,with a balance being Fe and impurities, wherein three or moreirregularities in a range of 0.2 μm to 8.0 μm in depth are present per100 μm in an interface between scale and a base iron, and tensilestrength is 1180 MPa or more.
 13. The hot stamp fonned body according toclaim 12, wherein an Si oxide, FeO, Fe₃O₄ and Fe₂O₃ are included in asurface of the hot stamp formed body, and a thickness of the scale is 10μm or less.
 14. The hot stamp formed body according to claim 12, whereinthe composition of the hot stamp formed body contains, in mass %, onekind or two or more kinds selected from a group consisting of Ni: 0.01%to 2.00%, Cu: 0.01% to 2.00%, Cr: 0.01% to 2.00%, Mo: 0.01% to 2.00%,Nb: 0.005% to 0.100%, V: 0.005% to 0.100%, and W: 0.005% to 0.100%. 15.The hot stamp formed body according to claim 12, wherein the compositionof the hot stamp formed body contains, in mass %, a total of 0.0003% to0.0300% of one kind or two or more kinds selected from the groupconsisting of REM, Ca, Ce and Mg.